The issue could be at your ankle, quad, or anterior hip.
Chances are it is a combination of tight tissue.
Have you done the ankle wall or pistol test?
These are 2 ways to see if you have full ankle range of motion or are missing critical corners.
The ankle wall lets you know where you come up short.
You can descend in a squat, but if you are missing ankle rom you hit a point(the wall) and you lose your balance causing you to fall backward or you compensate into a rounded position. You are at the limit of your ankle rom when you hit that spot.
Knees forward in the bottom position:
Motor control fix:
Load your hips and hamstrings as you initiate the squat.
Keep your shins vertical as long as possible.
If your knees go forward in the bottom position pull your knees back making your shins as vertical as possible.
Areas to target for mobilization:
Anterior chain (hip/quad)
Medial chain (adductors)
Posterior low chain (hamstring)
Posterior high chain (glutes)